An energy storage, e.g. used in electric vehicles, smart grids and active UPS (Uninterruptable Power Supply), normally requires a battery pack with parallel-connected battery strings to obtain suitable power ratings. For instance, if a battery string has a nominal voltage of 180 V and provides 20 Ah, thus having a power rating of 3.6 kWh, fifteen parallel-connected battery strings need to be provided to obtain an energy storage of 54 kWh.
Traditionally the battery pack (with the parallel-connected battery strings) is connected to a power converter circuit that converts the DC power stored in the batteries to AC power to a load. The power converter circuit may be implemented as an H-bridge, wherein the upper legs of the H-bridge each comprises several parallel-connected power switches connected to the battery pack, as illustrated in FIG. 1.
U.S. Pat. No. 7,248,490 discloses a system to convert DC power stored in multiple DC energy sources (such as battery strings) into AC power in a power grid application. A control circuit is provided to measure the power consumed in the power grid and activates an appropriate number of power converters to meet the need to perform peak shaving. Each energy source is assigned a dedicated DC/AC converter.
However, although the system disclosed in U.S. Pat. No. 7,248,490 relates to a power grid application in which individual battery strings may be connected or disconnected dependent on the actual need, the solution is expensive and requires an advanced controller.
U.S. Pat. No. 7,498,694 discloses a power management device for enabling multiple power sources to supply power to a load. The power management device includes a plurality of switches and a control logic. Each switch is coupled to a power source and the control logic is capable of selecting one of the power sources to provide power to the load by selecting the switches to cooperate in a time-divided fashion. Only one power supply is connected through a switch to the load at the same time, as disclosed in connection with FIG. 5.